The present invention relates to an insulation measurement method and an insulation measurement apparatus and, in particular, relates to the insulation measurement method and the insulation measurement apparatus which can detect the insulation state of a high-voltage power supply etc. accurately and reduce the influence of the voltage fluctuation of the high-voltage power supply by using the insulation measurement apparatus of a flying capacitor type.
Conventionally, a battery for charging electric power into and for supplying electric power to electrical equipments such as a light turning-on system and an air conditioner is mounted on an automobile. Automobiles of the day depend on electric power as is no exaggeration to say that the automobiles can not work without electric power.
That is, in the case where a suitable electric power supply can not be maintained, it becomes difficult to suitably control a vehicle. Further, there may arise a problem that an automobile stops if an alarm is issued or a driver overlooks this state. Thus, it has become more important to suitably monitor the power supply.
For example, as one of techniques for monitoring the power supply, there is a technique of determining the actual insulation state of the power supply placed in the insulation state. To be concrete, there is an insulation measurement circuit of a flying capacitor type (see JP-A-2004-170103).
Although the circuit configuration and the operation of the circuit in the technique disclosed in JP-A-2004-170103 will be explained in detail as a basic technique in an embodiment, the circuit configuration and the operation will be explained briefly. This circuit includes a capacitor in a state of being electrically floated from the grounding voltage, resistors and switching elements. As the capacitor, a ceramic capacitor capable of being miniaturized is used. The switching elements are suitably controlled so as to set the capacitor a charge power supply voltage (V0) corresponding to a charging time period of a voltage V of the power supply and the charge power supply voltage V0 is measured. Succeedingly, after the switching elements are suitably controlled to thereby discharge the voltage set to the capacitor, in order to measure a grounding resistor measurement voltage, the capacitor is charged by the power supply in a state that the one end terminal of the capacitor is coupled to a ground resistor and a voltage set to the capacitor at this time is measured (hereinafter called “a grounding resistor measurement voltage VC1a”). Then, after the grounding resistor measurement voltage VC1a is discharged, the switching elements are suitably controlled to thereby charge the capacitor in a state that the other end terminal of the capacitor is grounded via a ground resistor and a voltage set to the capacitor at this time is measured (hereinafter called “a grounding resistor measurement voltage VC1b”). When the measurement is completed, the grounding resistor measurement voltage VC1b is discharged. When these three kinds of voltages V0, VC1a and VC1b are measured, an insulation resistor conversion is performed based on these voltages in accordance with a calculation expression (VC1a+VC1b)/V0 and the insulation state is determined with reference to the conversion result and a predetermined table.
However, according to the technique disclosed in JP-A-2004-170103, the insulation state of the power supply is detected quickly and the variance of the characteristics of the capacitor is compensated to thereby realize a high detection accuracy. However, the aforesaid technique is realized based on that the voltage V of the power supply is constant. That is, there arises a case that the detection accuracy degrades when there is a difference of the charge voltage V0 of the power supply between the case of measuring the charge voltage V0 of the power supply set to the capacitor and the case of measuring the grounding resistor measurement voltages VC1a and VC1b. 